Synthesis and Characterization of Environmentally Responsive Core-Shell Hydrogel Nanoparticles
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Synthesis and Characterization of Environmentally Responsive Core-Shell Hydrogel Nanoparticles Clinton D. Jones, Christina Baker, and L. Andrew Lyon* School of Chemistry and Biochemistry, Georgia Institute of Technology Atlanta, GA 30332-0400, U.S.A. ABSTRACT We report the synthesis of environmentally responsive hydrogels as nano-sized particles with core-shell morphologies. Composed of co-polymers of N-isopropylacrylamide with various co-monomers, these materials can be designed to render the core and shell responsive to different stimuli or to different magnitudes of the same stimulus. The measured phase transitions reflect the degree to which the two materials interact and thereby modulate the responsivity of the particle as a whole. Characterization of these materials is accomplished via dynamic light scattering and electron microscopy. INTRODUCTION The behavior of sub-micron particles composed of environmentally-responsive hydrogels has become an active research area due to their potential applications in numerous fields.[1] Of these materials, the most widely studied are the temperature responsive poly(alkylacrylamides), specifically poly(N-isopropylacrylamide).[1-4] These particles typically display large-magnitude volume phase transitions in response to subtle changes in the temperature of their surroundings. The fundamental property-structure relationships have been investigated extensively, such that the magnitude, shape, position, and reversibility of the transition can be modulated by fairly trivial synthetic procedures. As the fundamentals of these microgels have become better understood, the interest in multifunctional or multiresponsive particles with more complex architectures has increased. Specifically, core-shell particles with spatially localized chemical functionalities or morphologies have been synthesized.[5-11] Many of the core-shell particles described previously contain both a responsive and nonresponsive component. In this paper, we present the synthesis and characterization of core-shell particles that are composed of two different types of responsive polymer. This has produced a multiresponsive (pH and temperature) core-shell hydrogel system that displays complex phase transition behavior. The effects of particle architecture and pH on the temperature-dependent swelling behavior are presented, as well as microscopic investigations into the resultant particle morphologies. EXPERIMENTAL DETAILS All reagents were purchased from Sigma-Aldrich. N-isopropylacrylamide monomer (NIPAm) was recrystallized from hexanes (J.T. Baker) and dried in vacuo prior to use. Acrylic acid (AAc), N,N’-methylenebisacrylamide (BIS), sodium dodecyl sulfate (SDS), and ammonium NN1.4.1
persulfate (APS) were all used as received. Water for all reactions, solution preparation, and polymer purification was distilled, purified to a resistance of 18 MΩ (Barnstead E-Pure system), and filtered through a 0.2-micron filter to remove particulate matter. Hydrogel nanoparticles were prepared via seed-and-feed aqueous free-rad
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